1. Field of the Invention
This invention relates to the polishing of semiconductor planar surfaces using a combined mechanical polishing and chemical etching treatment. More particularly, the present process relates to the rapid polishing of planar surfaces of III - V semiconductors to provide smooth, highly polished surfaces. A high rate of polishing is obtained with wafers and slices of III - V materials such as GaP, GaAs, GaAsP, InAs and the like. 2. Description of the Prior Art
A substantial volume of prior art has been generated in recent years relating to the chemical and mechanical polishing of semiconductor surfaces. As is well known, III - V compounds are used as the basic semiconductor portion of several types of devices, the most common being light emitting diodes. In many such devices, regular, smooth planar surfaces within close tolerances are required. In addition, the surfaces in question often must be characterized by substantial "flatness", or uniform surface angle with respect to a fixed reference plane.
Mechanical procedures and apparatus for polishing planar semiconductor surfaces are known in the art. For example, the semiconductor wafer or slice is attached to a mounting block, carried by a freely rotatable spindle. Then the semiconductor surface is pressed against a moving polishing surface, such as a polyurethane polishing cloth carried by a driven turntable. A commercially available device which can be used to mechanically polish semiconductor wafers or slices is the Geoscience PA-100 laboratory polisher available from Geoscience, Inc. of Mount Vernon, N.Y.
In order to rapidly polish semiconductor devices chemical etchants are employed in combination with mechanical polishing. The chemical etchant can be selected to react with the semiconductor surface to form a product more easily removed from the surface by the mechanical polisher. However, the etchant is often not selected from those which strongly attack the semiconductor compounds since an undesired rough or pitted surface could result.
Chemical etchant systems are described in the following publications and patents:
The Journal of the Electrochemical Society, 109, 880 (1962) contains an article by Fuller & Allison entitled "A Polishing Etchant for III - V Semiconductors" describing the employment of an etchant consisting basically of an organic liquid in which a halogen, generally Cl.sub.2 or Br.sub.2, is dissolved. Methyl alcohol and glacial acetic acid in a "dry" state are described as preferred organic liquids. Fuller and Allison state "adding water slows the etching reaction." The etchant is described as being suitable for GaAs and GaP crystals. For GaP, Br.sub.2 is noted as being "too slow", the best procedure being to immerse the specimen in methyl alcohol through which chlorine gas is slowly bubbled. This reference also states that aqua regia (nitrohydrochloric acid) is an etchant for the III - V surfaces but is undesirable for a number of reasons. Organic solutions of bromine or chlorine are recognized by the artisan as presenting hazards due to the possibility of violent reaction.
Oldham, "Chemical Polishing of GaP" at page 57 of the Jan.-Feb. 1965 Electrochemical Technology, describes a mechanical-chemical polishing technique employing a viscous mixture of a solution of methanol saturated with chlorine and an equal volume of H.sub.3 PO.sub.4. The GaP sample is attached to a Teflon disk which glides against a glass disk immersed in the etchant.
Reisman and Rohr, "Room Temperature Chemical Polishing of Ge and GaAs". Journal of the Electrochemical Society, Vol. III, No. 12, 1 and 25, employ NaOCl in aqueous solution to etch the semiconductor surfaces in combination with mechanical polishing techniques. The reaction of NaOCl with the chloride in an acidified solution to form free chlorine and water is recognized as an analytical tool to determine available chlorine content.
Basi U.S. Pat. No. 3,775,201 is directed to mechanical-chemical polishing of GaP wafers with alkali metal oxybromide with or without a base. The etchant employed is described as being specific for GaP surfaces. The base is described as a solvent for the GaPO.sub.4 formed by reaction of GaP with NaOBr. Standard polishing equipment is described as being illustrated in U.S. Pat. No. 3,342,652. Basi '201 states that the process described in Basi Pat. No. 3,738,882 is inoperable for polishing GaP.
U.S. Pat. No. 3,738,882 to Basi is directed to a semiconductor mechanical-chemical polishing process specific for GaAs. The chemical etchant is a combination of sodium hypochlorite and sodium carbonate in aqueous solution. The chemical reaction involving hypochlorite is stated by Basi to be as follows: EQU GaAs + 4NaOCl .fwdarw. GaAsO.sub.4 + 4NaCl.
The U.S. Pat. No. 3,738,882 patent recognizes the prior art of U.S. patent No. 3,342,652 to Reismann et al as disclosing the use of aqueous sodium hypochlorite and potassium hypochlorite solutions as oxidizing agents in the polishing of gallium arsenide. Basi states "This teaching only discloses the use of very dilute solutions of sodium and potassium hypochlorite, which are useful for polishing gallium arsenide, and illustrates that higher concentrations are deleterious in that they produce oxidized and pitted surfaces upon the wafer or slice." "It has been reported that higher concentrations" (above the 1.25 to 2.4 grams per liter described for U.S. Pat. No. 3,342,652) "of sodium hypochlorite produce pitted surfaces and the polishing rate even using dilute solutions is only in the magnitude of 0.7 to 2 mils per hour."